Page 30 - Reservoir Formation Damage

14 Reservoir Formation Damage

Montmorillonite) group, and (3) Illite group. In addition, there are mixed-
layer clay minerals formed from several of these three basic groups
(Weaver and Pollard, 1973).
The description of the various clay minerals of the sedimentary for-
mations is given by Degens (1965, p. 16). The morphology and the major
reservoir problems of the various clay minerals is described in Table 2-2
by Ezzat (1990).
Readers are referred to Chilingarian and Vorabutr (1981), Chapters 5
and 8, for a detailed review of the clays and their reactivity with aque-
ous solutions.

Mineral Sensitivity of Sedimentary Formations
Among other factors, the interactions of the clay minerals with aque-
ous solutions is the primary culprit for the damage of petroleum-bearing
formations. Amaefule et al. (1988) state that rock-fluid interactions in
sedimentary formations can be classified in two groups: (1) chemical
reactions resulting from the contact of rock minerals with incompatible
fluids, and (2) physical processes caused by excessive flow rates and
pressure gradients.

Table 2-2
Typical Problems Caused by the Authigenic Clay Mineral

Surface Area
2
Mineral m /gm* Major Reservoir Problems
Kaolinite 20 Breaks apart, migrates and concentrates at
the pore throat causing severe plugging
and loss of permeability.
Chlorite 100 Extremely sensitive to acid and oxygenated
waters. Will precipitate gelatineous Fe(OH) 3
which will not pass through pore throats.
Illite 100 Plugs pore throats with other migrating
fines. Leaching of potassium ions will
change it to expandable clay.
Smectite 700 Water sensitive, 100% expandable. Causes
loss of microporosity and permeability.
Mixed 100-700 Breaks apart in clumps and bridges across
Layer pores reducing permeability.
t After Ezzat, ©1990 SPE; reprinted by permission of the Society of Petroleum Engineers.
* After David K. Davies—Sandstone Reservoirs—Ezzat (1990).